A polyethylene filament which is produced from an ultra-high molecular weight polyethylene by a so-called gel-spinning method and which has higher strength and a higher elastic modulus than conventional filaments is disclosed in JP-B-60-47922, and this filament has already come into industrially wide use. This high strength polyethylene filament has advantages in its high strength and high elastic modulus. However, the high elastic modulus thereof sometimes induces disadvantages in various applications. For example, in cases where the high strength polyethylene filament is used for ordinary cloth, the resultant cloth is very stiff to the touch and thus very unsuitable in view of comfort in wearing. In cases where the high strength polyethylene filament is used for a bulletproof vest, it is demanded that the bulletproof vest should be made of a plurality of pieces of cloth superposed on one another so as to confront dangers which recently have been escalated more and more. As a result, the thickness of the cloth composing the vest is increased, so that one can not freely move in such a vest.
Under such circumstances, a filament which has a lower mass (METSUKE) and a very high strength is demanded.
In the meantime, a variety of olefin-based filaments and films recently have been used for separators for various batteries. In cases where high strength polyethylene filaments are used as non-woven cloth or reinforcing materials for such separators, the high strength polyethylene filaments to be used are required to have such properties that can provide non-woven cloth with thin mass (METSUKE) and concurrently with a high strength maintained, in order to meet a demand for further compacting batteries.
JP-B-64-8732 discloses a filament which is made from an ultra-high molecular weight polyethylene as a starting material by so-called “gel spinning method” and which has a lower fineness, a higher strength and a higher elastic modulus than any of conventional filaments. However, the above production of the high strength polyethylene filament with a lower fineness by the gel spinning method uses a solvent, and the use of a solvent has a disadvantage of causing fusion of the filaments. Particularly in case where a very fine filament is desired, the drawing tension tends to increase with an increased spinning tension, which induces the fusion of filaments.
Japanese Patent No. 3034934 discloses a high strength polyethylene filament having a fineness of 16.7 dtex or less as a monofilament, which is produced by drawing a high molecular weight polyethylene having a weight-average molecular weight of 600,000 to 1,500,000. The fineness of the monofilament achieved in this patent is 2.4 dtex at least, and a high strength polyethylene filament having a fineness of 1.5 dtex or less which the present invention has achieved can not be obtained.
A high strength polyethylene filament produced by melt spinning is disclosed in, for example, U.S. Pat. No. 4,228,118. According to this patent, the high strength polyethylene filament disclosed has a strength of 17.1 cN/dtex, an elastic modulus of 754 cN/dtex, and a finness of 2.0 dtex at least as a monofilament of the fiber. Thus, a high strength polyethylene filament having a fineness of 1.5 dtex or less has not yet been obtained by the melt spinning.
One of commercially available polyethylene filaments made by the melt spinning has a tensile strength of about 10 cN/dtex at most, even though it is classified to high performance polyethylenes. At present, a polyethylene filament having a strength of as high as 15 cN/dtex or more has not yet been manufactured and put on the market.
The most effective solution to satisfy such a wide range of requirements is to decrease the fineness of a monofilament while maintaining the strength of the filament. However, the fineness of the monofilament of a polyethylene filament obtained by the melt spinning having a strength of as high as 15.0 cN/dtex or more is generally 2.0 to 5.0 dtex. Thus, it is impossible in a practical view to obtain as in the present invention not only a polyethylene filament which has a fineness of as low as 1.5 dtex or less, but also a polyethylene filament having a fineness of so far low as 1.0 dtex, at a productivity high enough for industrial production, even though such a filament can be present in a moment. Even if such a filament can be produced, the physical properties of the resultant filament markedly degrade and thus, this filament is insufficient for practical use. On the other hand, a high strength polyethylene filament having a fineness of as low as 0.5 dtex or less can be obtained by the gel spinning. However, such a high strength polyethylene filament with a lower fineness has problems in that there are many fusing points among each of the monofilaments thereof, and that it is very hard to obtain a desired uniform filament having a low fineness.